1. Field of the Invention
The present invention relates to means for controlling fluid flow through a flow conduit. In a specific application, the present invention relates to self-closing safety valves employed in the production tubing of a completed well to prevent uncontrolled fluid flow through the tubing following failure of or damage to the restraining structure at the wellhead.
2. Description of the Prior Art
In many wells producing from a high pressure, subterranean petroleum bearing formation, a self-closing safety valve is positioned in the production string when the well is initially completed to prevent uncontrolled flow through the well in the event the confining or restraining structure at the wellhead should be destroyed or become inoperative. In some cases, however, wells were not equipped with subsurface safety valves when the production tubing was installed and regulatory agencies have required that production from some of these wells be terminated until safety valves are installed.
Where the need for a downhole safety valve is known in advance, a suitable nipple is usually included in the tubing string as the string is initially positioned in the well. Thereafter, a safety valve is lowered into the tubing string and special dogs or other means on the valve spring into receiving grooves or the like formed in the nipple to hold the valve in operative position. In some cases, the wells are initially equipped with non-retrievable, subsurface safety valves which form a part of the tubing string itself. In those systems where the valve is a part of the tubing string itself, inspection, repair or replacement of the valve may often require a complete shut down of production while the tubing string is removed from the well.
Where the tubing string lacks special landing nipples for receiving the required safety valve, it is often necessary to withdraw the tubing string from the well, insert the required nipple and then replace the string. This procedure is undesirable primarily in that it may involve a significant loss in production time.
Conventional safety valves often employ relatively narrow flow passage openings which form a restriction to the flow of fluids through the well conduit. Restricted flow passages are normally undesirable, particularly where the formation pressure is relatively low, since the restriction limits production below that which would otherwise be possible. Small valve openings are also undesirable in that it is difficult or impossible for equipment to be lowered through the valve opening so that testing or other procedures may be conducted below the valve. Another shortcoming associated with restricted valve openings is the tendency of such valves to be cut away by abrasive materials carried in the petroleum fluids flowing through the valve. This is particularly evident where the valve closure surfaces are directly exposed to the fluid flow.
Conventionally, the sensing elements employed to close safety valves have included either flow sensitive or pressure sensitive devices. In these devices, an increase in flow rate or a decrease in pressure respectively indicate uncontrolled well flow which automatically triggers closing of the safety valve. Flow sensing elements are undesirable to the extent that the required flow monitoring element may restrict flow. In marginal wells having a relatively low bottomhole pressure, any restriction to flow can limit production below that which would otherwise be obtainable.
The prior art is replete with bellows driven pressure sensitive controls employed in conjunction with self-operating valving mechanism. In such prior art controls, bellows damage is often encountered because of extreme pressure differentials imposed across the bellow walls causing the bellows to be distorted beyond design limits. Another problem associated with conventional bellows operated controls stems from the fact that relatively slow, low force closing movements obtainable with the bellows may cause partial or improper seating of the valve.